1. Field of the Invention
The present invention relates to a light guide plate for liquid crystal display (LCD) back light units and an LCD back light unit using the same. More particularly, the present invention relates to a light guide plate for LCD back light units, including front prisms and dot prisms formed on front and rear faces, respectively, to increase the brightness and uniformity of light output therefrom, and an LCD back light unit using the same.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) device is a parallel plate capacitor with a dielectric, here a liquid crystal (LC) material, between the plates. A LC material has a liquid form and crystal molecular structure. The LCD also includes multiple selectable electrodes that apply an appropriate electric field to the LC material, allowing figures or images to be displayed.
The LCD device is a passive display device, i.e., it cannot emit light by itself. Therefore, a back light unit that provides light needs to be adjacent the LCD device. Light emitted from the back light unit is provided to a panel of the LCD device having the LC material uniformly arranged therein, where the amount of the light transmitted is controlled to display images. Based on the position of the light source for emitting light, LCD back light units may be classified based on the position of the light source relative to the LCD panel. In a direct-type back light unit, the light source is disposed directly beneath the LCD panel. In an edge-type back light unit, the light source is disposed at the side of the LCD panel.
When an edge-type back light unit is used, light may not be uniformly transmitted over the entire area of the back light unit. Specifically, the edge of the back light unit adjacent the light source may be brighter than other areas of the back light unit. In order to prevent this phenomenon, a light guide plate may be used.
The light guide plate may convert linear light output from the light source, received at a side face of the light guide plate, into planar light, and may output the planar light from a front face of the light guide plate. Specifically, the light guide plate may uniformly project light emitted from the light source onto the front face of the light guide plate. However, when the light source is adjacent the side face of the light guide plate, the front face of the light guide plate may not be uniformly bright, but light may be concentrated at the front face away from the side surfaces.
To counter this effect, a rough surface, having a predetermined shape and location in accordance with the type and locations of the light sources, may be provided at the rear surface of the light guide plate to diffusely reflect light incident thereon, allowing light to be uniformly emitted from the entire surface of the light guide plate. As a result of the diffuse reflection, planar light having relatively high brightness and uniformity of light over the panel of the LCD device may be obtained.
However, the light intensity may now be higher at areas corresponding to the rough surface than the remainder of the rear face. This effect may be particularly pronounced as panel size increases, since the amount of light reaching the area far away from the light source may be insufficient, resulting in low light intensity in the area far away from the light source.
In order to solve the above-mentioned problems, i.e., in order to increase the uniformity of the light, a diffuser sheet may be disposed at the front surface of the light guide plate, and may uniformly diffuse light output from the light guide plate. However, after light has been diffused horizontally and vertically by the diffuser sheet, brightness of the light may be sharply decreased. Thus, a prism sheet may be provided at the front surface of the diffuser sheet, and may refract and condense light to increase brightness.
However, the use of the diffuser sheet and the prism sheet increase the manufacturing costs of the back light unit. Therefore, there is still a need for a more practical back light unit that outputs planar light having excellent visibility, high brightness and high uniformity.